ACTIVATED CARBON AND HYDROGEN ADSORPTION STORAGE

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2 ACTIVATED CARBON AND HYDROGEN ADSORPTION STORAGE L. L. VASILIEV, L. E. KANONCHIK, А. G. KULAKOV, D. A. MISHKINIS Laboratory of Porous Media Luikov Heat & Mass Transfer Institute, National Academy of Sciences, P. Brovka, 15, 2272, Minsk, Belarus Phone/Fax: (375-17) ; Abstract Activated carbons were chosen as an efficient hydrogen sorption materials to design gas storage systems. Based on experimental data empirical dependences for choosing commercially available carbon hydrogen sorbents systems were proposed. To increase gas sorption capacity technology of carbon additional activation was applied. Different heat pipe devices for sorbent bed thermal control were developed for integrating in sorption storage vessels. Developed theoretical model of cylindrical AG vessel with internal finned heater (HP based) can be used for storage systems designing. The designed AGS systems and modified carbon sorbents are perspective for the hydrogen storage and for two-fuel automobile Keywords: Carbon, hydrogen, sorption, heat pipe, adsorption storage 1. Introduction From environmental point of view hydrogen is the cleanest known fuel, and from economic point of view hydrogen technology will be able to revolutionize the transport and energy market. Now hydrogen becomes the real alternative for fossil fuel systems. Among the advantages of hydrogen are its low density and small volume heat of combustion. But recently none of the existing methods of hydrogen storage is efficient in terms of energy density, of neither a volume nor mass and gas release rate. From the application point of view it is vital to find the most effective way to store hydrogen and then, to replace the current fuel systems. Hydrogen storage by solid materials is the most recent system proposed [1]. Initially the research was based on cryogenic systems [2], now the studies have focused in the search of the high capacity adsorbent to be used at room temperature. Due to the high density of the adsorbed phase (it corresponds to density of pressurized gas at 6 8 MPa) the gas storage capacity in the vessel with sorbent can increase significantly. Since it is close to the liquid density, the volumetric capacity of adsorption system is predicted to be higher then for compression system. Activated carbons, activated carbon fibers and graphite nanofibers are perspective candidates for hydrogen adsorption storage. Many metals, alloys and intermetallic compounds can also reversible adsorb large amounts of 1

3 hydrogen. However, none of them is known practically effective for the mobile storage applications. The objective of this work is to analyze hydrogen storage in several porous carbonbased materials with different porous structures to propose perspective activated carbons (carbon fibers) for high performance hydrogen storage systems, to suggest methods of its hydrogen adsorption properties improvement. Another interrelated work objective is development of thermally regulated adsorption storage system for dual-fuel (hydrogen and natural gas) automobile. In fact, such gases as methane, ammonia, methanol, etc can be considered as hydrogen storages by itself also. 2. Carbon materials as a storage medium for gases Activated carbon is well known as one of the best adsorbents for gases [3]. In contrast to the chemisorption in metal hydrides [4], the phenomenon of physical adsorption is essentially accumulation of the undissociated hydrogen molecules on a surface of microporous carbon fibers or particles. This property is due to ability of carbon to be prepared in a very fine powdered or fiber form with highly porous structure and due to specific interactions between carbon atoms and gas molecules. The total amount of adsorbed hydrogen strongly depends on the pore geometry and pore size distribution as well as the storage pressure and temperature. Recently many improvements have been accomplished to obtain microporous carbonaceous materials with extremely high adsorbing properties for different gases [5]. Adsorption of methane and hydrogen are usually takes place in micropores. Macropores have no practically influence on the adsorption capacity, as they are only important for the gas compression and for adsorption/desorption reaction rates [6]. Due to its high surface area and abundant pore volume, activated carbon is considered as good adsorbent. For conventional activated carbon, the hydrogen uptake is proportional to its surface area and pore volume, while, unfortunately, a high hydrogen adsorption capacity (4 ~ 6 wt%) can be only obtained at very low cryogenic temperatures or very high pressures. To gain the goal of the problem efficient methane and hydrogen storage and transportation it is necessary to develop a high performance microporous adsorbent material and an advanced system of the vessel thermal control. Thus, the cheap activated carbon fabricated by special thermal treatment of impregnated raw (wood, sawdust, cellulose, straw, paper for recycling, peat etc.) is attractive for modern sorption technologies. The use of specific organic and non-organic compounds as raw impregnates offers production activated carbons with controlled porous structure and high yield (up to 5 wt %). Developed advanced technology allows to produce the homogeneous carbon adsorbents with benzene pore volume,3,6 cm 3 /g (7 8 % - volume of micropores), nitrogen surface area up to 15 m 2 /g, iodine adsorption capacity 4 7 wt % and methane adsorption capacity up to 16 mg/g (3.5 MPa, 293 K). Impregnated cellulose - containing raw for manufacture special activated carbon materials for ammonia, methane and hydrogen storage systems with high microporosity, surface area and narrow micropore size distribution is the attractive host material for adsorption of different gases. The activated carbon fiber Busofit and activated wood-based carbon particles ([7], the Institute of General and Inorganic Chemistry, NASB) fabricated in Belarus are perspective materials for gas storage systems (Figs. 1 2). 2

4 b) a) Figure 1. Active carbon material made from waste wood (IGIC NASB): a) Image multiplied by 3 times; b) by 1 times b) a) Figure 2. Active carbon fiber Busofit : a) Image multiplied by 5 times; b) by 1 times Busofit is a universal adsorbent, which is efficient to adsorb different gases (H2, N2, O2, CH4, and NH3). Figure 2 shows the texture of the active carbon fiber filament. The carbon fiber refers to microporous sorbents with a developed surface and a complicated bimodal structure. The material can be performed as a loose fibers bed or felt or as monolithic blocks with binder to have a good thermal conductivity along the filament. In our experiments some samples of activated carbon Busofit obtained by the new technology were investigated. The surface area of the commercially available Busofit was measured with BET Sorbtometer NOVA and varied from 114 m2/g up to 157 m2/g. Now it is clear, that methane and hydrogen storage vessels filled with Busofit have certain advantages (for example, methane storage capacity near 17 v/v). To be commercially profitable the adsorption storage is required to have at last 15 v/v. It can be considered as a typical microporous adsorbent with pore diameter near 1 2 nm and at the same time as material with high gas permeability. The micropore distribution is performed mostly on the carbon filament surface. Nowadays a program was undertaken to examine the parameters of an active carbon 3

5 fiber to optimize both the mass uptake of ammonia, methane and hydrogen and the carbon density. Busofit has such advantages as - high rate of adsorption and desorption; - uniform surface pore distribution ( nm); - small number of macropores (1-2 nm) with its specific surface area.5-2 m 2 /g; - small number of mesopores with its specific surface area 5 m 2 /g. 3. Experimental research of texture and hydrogen-sorption capacity for the activated carbon materials Activated carbon fibers (commercially available Busofit-AYTM and modified grades of Busofit-AYTM ) and granular activated carbon (commercially available and additionally activated at Luikov Institute Porous Media Laboratory) have been used in this work (Table 1). TABLE 1. Textural characteristics and hydrogen-sorption capacities at 77 K and.1 MPa for the researched carbon materials No Sorbent a v, ml/g a H, wt% S H, m 2 /g S BET, m 2 /g S DR, m 2 /g V DR, ml/g V t, ml/g R DR, Ǻ 1 Busofit Busofit-М Busofit-М Busofit-М WAC WAC WAC С Norit sorbonorit Sutclife Remarks: WAC wood-based active carbon; a v volume capacity of hydrogen storage using physisorption, ml/g; a capacity of hydrogen storage using physisorption, wt%, g/1 g; S H BET surface area determined on hydrogen, m 2 /g; S BET BET surface area determined on nitrogen, m 2 /g; S DR surface area, determined on Dubinin- Radushkevich method, m 2 /g; V DR micropore volume, determined on Dubinin- Radushkevich method, ml/g; V t mesopore volume, determined on t-method, ml/g; R DR size of pore, determined on Dubinin-Radushkevich method, Ǻ 4

6 Porous texture of the different materials was all characterized using nitrogen (N 2 ) physisorption at 77 K and up to a pressure of.1 MPa. From the nitrogen physisorption data, obtained with the High Speed Gas Sorption Analyser NOVA 12, the BETsurface area, total pore volume, microporous volume and t-volume were derived. The BET surface area (S BET ) is the surface area of the sorbent according to the model formulated by Brunauer et al. [8] for planar surfaces. The micropore volume is defined as the pore volume of the pores < 2 nm. Microporous volumes calculated from the application of the Dubinin-Radushkevich equation to the N 2 adsorption isotherms at 77 K. The mean pore size of each sample obtained from N 2 adsorption was determined by applying Dubinin-Radushkevich equation. The hydrogen sorption isotherms were measured with the High Speed Gas Sorption Analyser NOVA 12 at 77 K in the pressure range.1 MPa. Table 1 summarizes the results of the N 2 and H 2 physisorption measurements of the materials analysed. All samples are highly micro-and mesoporous carbon materials. In our experiments four samples of carbon Busofit-AYTM (1 4) and three samples of wood-based activated carbon (5 7) obtained by new technology were investigated. The activated carbon 27C (8) is made in the Great Britain from coconut shell. Samples 9 and 1 granular activated carbons, specially developed for effective storage of methane. According to the offered technology some samples of Busofit AYTM have been prepared with additional activation using thermal processing at high temperature 85 C. In this way some of the carbon atoms are removed by gasification, which yields a very porous structure. Numerous pores were formed in the carbon material increasing a specific surface area due to the growth of micropore volume. Additional activation of a sample 1 was carried out at the presence of oxygen. As follows from Table 1 the increase of time of activation from two hours until eight hours in an atmosphere of carbon dioxide gas promotes increase to sorption capacity almost in 1.5 times (samples 2 and 4). The atmosphere of carbonic gas appeared more preferably oxygen for growth of a specific surface and sorption capacity time of activation of samples 1 and 2 was identical. To increase the adsorbent capacity and the bulk density of material we compressed active carbon fiber together with a binder. Briquet Busofit disks have a high effective thermal conductivity and a large surface area. Wood-based carbons (5 7) were produced by controlled pyrolysis of waste wood by the Institute of General and Inorganic Chemistry NASB and additional activation (WAC 3-). As seen in Table 1, the greatest values of a surface area and micropore volume among carbon fibrous materials has Busofit-M8 ; among wood-based activated carbons it is stand out WAC 3-, granular carbons Sutcliff. The approach of research institutes AGLARG [9] was used for an operative estimation of gas sorption capacity for carbon sorbents. According to it micropore volume and the specific surface area have been chosen as determining parameters. To obtain the function approximating dependence of hydrogen sorption capacity on carbon materials from value of a specific surface area (at pressure.1 MPa and temperature 77 K), we used our experimental data (Table 1) and an experimental database (Table 2) of group of institutes - Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University [1]. 5

7 A test matrix of about 2 different carbon samples, including commercial carbon fibers and fiber composites, graphite nanofibers, carbon nanowebs and single walled carbon nanotubes was assembled. The sorbents were chosen to represent a large variation in surface areas and micropore volumes. Both non-porous materials, such as graphites, and microporous sorbents, such as activated carbons, were selected. Characterization via N 2 adsorption at 77 K was conducted on the majority of the samples; for this a Quantachrome Autosorb-1 system was used. The results of the N 2 and H 2 physisorption measurements are shown in Table 2. In the table CNF is used to designate carbon nanofibers, ACF is used for activated carbon fibers and AC for activated carbon. TABLE 2. Textural characteristics and hydrogen-sorption capacities at 77 K and.1 MPa for carbonaceous materials [1] No Sorbent S BET, m 2 /g V DR, ml/g а v, ml/g а v, meso, ml/g а v, micro, ml/g 1 Synthetic graphite 7. 2 Large-diameter CNF Activated graphite Medium-diameter CNF1 5 ACF ACF AC Norit AC Norit ROZ Activated graphite Medium-diameter CNF2 11 AC Norit SX ACF AC Norit UOK A AC Norit SX AC Norit SX 1G AIR AC Norit GSX AC Norit SX plus AC Norit SX 1 G AC Norit AC Norit Darco KB Hyperion CNF For carbon samples we have found linear relationship between BET surface area and volume sorption capacity of hydrogen at 77 K and.1 MPa as: av.783 SBET 84.2 (1) Figure 3 shows all experimental data and variants of approximation. It is possible to see that our linear correlation fairly good describes all results except for the literature 6

8 data which correspond to materials with low values hydrogen sorption capacity (а v < 5 ml/g). Influence of micropore volume on sorption capacity of hydrogen for various carbon materials under the chosen conditions (77 K,.1 MPa) is well described by the following linear correlation: av, ml/g a V (2) v DR SBET, m 2/g Figure 3. Volume capacity of hydrogen storage for carbon sorbents vs. BET surface area at pressure.1 MPa and 77 K: experimental data (Table 1), a continuous line the linear approximation obtained by authors; experimental data (Table 2), a dashed line the linear approximation given in [1]. From Fig. 4 it can be concluded that this correlation does not apply to the carbon samples with low values sorption capacity of hydrogen (а v < 5 ml/g). "Hydrogen" sorbent should have maximum high value of a specific surface area and maximum narrow distribution of micropores. av, ml/g VDR, ml Figure 4. Volume capacity of hydrogen storage for carbon sorbents vs. micropore volume (determined on Dubinin-Radushkevich method) at pressure.1 MPa and 77 K: experimental data (Table 1), a continuous line the linear approximation obtained by authors; experimental data (Table 2), a dashed line the linear approximation given in [1] 7

9 As follows from Table 1, sorbents "Busofit-M8", "Sutcliff" have micropore volume more than 1 ml/g. They are also the best storage systems for hydrogen (accordingly 253 ml(stp)/g and 237 ml(stp)/g) due to physisorption. Our results demonstrate that a large capacity of adsorbed hydrogen by physisorption under chosen conditions is obtained with sorbents containing a large volume of micropores and a high BET surface area. Optimization of sorbent and sorption conditions is expected to lead to storage capacity of 5 6 ml(stp)/g, close to targets set for mobile applications. 4. The analysis of isotherms of hydrogen sorption by carbon materials at medium pressures 4.1. Experimental set-up The analysis of isotherms of gas at the temperature interval K and pressure interval.1 6 MPa was realized by the gravimetric control of the sample (2 4 g) during adsorption/desorption cycle. The experimental set-up is shown in Fig. 5. Figure 5. Experimental apparatus: 1 AG cylinder, 2 electronic balance, 3 insulated chamber, 4 heat exchanger, 5 thermostat, 6 computer with software, 7, 1 pressure sensors, 8, valves, 9 calibrated volume, 11 vacuum pump, 12 hydrogen vessel, 13 helium vessel, 14, 15 reducer, 2 flow meter, 21,23 thermocouples, 22 fans, 24 sorbent bed, 25 heat exchanger 8

10 The full amount of the adsorbate was measured. The cylindrical experimental rig of 47 mm inner diameter and 54 mm length was used to simulate full-scale conditions of the experiment. This experimental rig was made from the stainless steel and served as a simulator of the real vessel in the ratio 1:5. Thermocouples were attached to the sample through the sorbent bed (12.5 mm thick). Pressure sensor was used for pressure measurements in the experimental chamber Sorption isotherms for a high surface activated carbons The rate of the adsorption/desorption of different gases (methane, hydrogen) on the surface of Busofit can be evaluated by the isotherms analysis at different temperatures of the sorbent bed. In order to study the sorption capacity of the adsorbent it is necessary to know the quantity of gas adsorbed on each point of the cycle. There is a general need to have a good fit of experimental isotherms and temperature and to extrapolate some isotherms beside the experimental field. For the carbon fiber Busofit the approach of Dubinin is well adapted and allows linking quite simply the physical properties of Busofit to the capacity of adsorption of the carbon fiber. Methane isotherms evolution during the cycle of adsorption/desorption of Busofit AYTM is presented in [11, 12]. Based on these data we can conclude that Busofit-AYTM is competitive to best activated carbons with methane adsorption capacity g/g at 273 K. As an example, Fig. 6 shows the experimental isotherms of the adsorption and desorption of hydrogen on carbon fiber Busofit-M8 and wood-based activated carbon WAC 3- at the nitrogen temperature. Absence of an appreciable hysteresis confirms that reversible physisorption exclusively takes place with all investigated materials. av, ml(stp)/g Bus-M8. ads. Bus-M8. des. WAC 3- ads. WAC 3- des.,,4,8 1,2 P, MPa Figure 6. Isotherms of hydrogen adsorption and desorption on carbon materials at temperature 77 K, measured by the High Speed Gas Sorption Analyser NOVA 12 Physisorption on microporous carbons can be described with the Dubinin- Radushkevich equation [13] 9

11 2 W ln / RT Psat P. (3) aeq exp va E The theory of microporous volume filling, worked out by Dubinin, is widely used for quantitative characteristic of adsorptive properties and basic varieties of porous structure. Equilibrium state equation (3) includes the saturation pressure P sat. Since the hydrogen sorption isotherms are measured within the temperature and pressure intervals comprising the regions of supercritical states of the adsorptive ( Tcr K Pcr МPа ), the notion of saturation pressure loses its physical meaning. In the work [14] the saturation pressure is determined by the formula: 2 Psat Pcr ( T / Tcr ) (4) As a results of the experiments, we obtained hydrogen sorption isotherms for different carbon materials and empirical coefficients for the Dubinin-Radushkevich equation (5), presented in Table 3. TABLE 3. The empirical coefficients of Dubinin-Radushkevich equation for hydrogen sorption on the carbon materials Material W, ml/g E, J/g Busofit-М Busofit-М Busofit-М Sutclife WAC С With the purpose of a choice of the most suitable sorbent of hydrogen, isotherms (Fig. 7) of studied materials were compared at a level of pressure MPa, representing practical interest for development of onboard storage system. It is obvious, that the best sorbent in the field of nitrogen temperature the activated fiber "Busofit- M8", having major effective porosity (.78), bulk density of 5 kg/m 3 and an advanced surface area (2985 m 2 /g). At pressure 6 MPa "Busofit-M8" reaches storage capacity up to 482 ml/g. Apparently, that 8-hours post-treatment of a carbon fiber by carbonic gas enhanced both microstructural, and the surface characteristics of the original "Busofit". As a result, the hydrogen sorption of a carbon fiber at cryogenic temperatures was improved almost twice, in comparison with samples "Busofit-M2" and "Busofit -M4" whose activation was prolonged a smaller time. From the analysis of physisorption isotherms (Fig. 7) follows, that the wood-based carbon WAC 3- with the greater specific surface area (S BET =1382 m 2 /g) immerses the smaller amount of hydrogen and its isotherm lays below an isotherm of carbon 27C with a specific surface area equal to 13 m 2 /g. The similar 1

12 "abnormal" behaviour can be explaining different pore size distributions of carbon materials [14]. a v, ml(stp)/g a, wt% a v, ml(stp)/g P, MPa Bus-M8 Sutcliff Bus-М4 Bus-М2 27 С 153 К 193 К 293 К WAC Figure 7. Hydrogen sorption isotherms for the temperature 77 K and different carbon materials: 1 Busofit-М8, 2 Sutcliff, 3 Busofit- М4, 4 Busofit-М2, 5 27 C, 6 WAC 3- Figure 8. Volume capacity of hydrogen storage using physisorption at pressure 6 MPa for active carbon materials and different temperatures A summary of the results on volumetric capacity of storage of hydrogen for the investigated materials is given on Fig. 8. It is observed that the H 2 uptakes are very low at room temperature and pressure 6 MPa for all carbon samples, but the wood-based material of large surface area has the best sorption characteristics storage capacity equal of 8 ml(stp)/g. The influence of temperature can be seen on Figs The storage capability is increasing for lower temperatures. Figure 9 compares the behaviour of the adsorption isotherms at different temperature levels for two of the more promising samples: steam activated Busofit-M8 and wood-based carbon WAC 3-. The shape of the isotherms in the two cases is dissimilar. The isotherms for the 77 and 153 K exhibit a classical type 1 isotherm shape indicating a microporous material. The isotherms at room temperature exhibit a much less pronounced curvature (more like type II isotherm). As is seen from plots (Fig. 9) experimental data fit the calculated adsorption values (Dubinin- Radushkevich equation) with an error sufficient for practical purposes. 11

13 a v, ml(stp)/g a v, ml(stp)/g P, MPa P, MPa a) b) Figure 9. Hydrogen adsorption isotherms for active carbon fiber Busofit-M8 (a), wood-based cardon WAC 3- (b) and different temperatures (1 77, 2 153, 3 193, К): experimental data points, calculated data (Dubinin-Radushkevich equation) lines 5. Heat and mass transfer in the sorbent bed Carbon material can be used as a compact "sandwich" with cylindrical or flat heat pipes, applied as thermal control systems. The dynamic mathematical thermal model of the sorbent bed (Fig. 1) has following constituents [12]: 1) Dubinin and Radushkevich equation (3) of the state of gas; 2) the equation of energy: T T T T a r ccg C aca rcvcg r r rqst, (5) r r r z z where the isosteric heat of desorption is: q 3) the equation of continuity: 4) the equation of kinetic of sorption: st ln P R T T ln / a const ; (6) r c a rcv ; (7) z da K d so E exp a R T eq, (8) a 12

14 Figure 1. Diagram of the calculated element of the SSH: 1 fin; 2 sorbent; 3 shell of the heating element; 4 channel for gas discharge formed by the perforated tube and the cylinder body; 5 cylinder body where 2 K s 15Ds / R p, R p - radii of the particle. D s constant necessary to determine the coefficient of a surface diffusion, Ds Ds exp E/ R T The solution was found for the fixed gas flow from the SSH vessel with boundary conditions: P P d L r c a rdrdz g / N (9) d r T r, z T r, z T ; env ; (1) T T T z z, T z z S ; T, (11) env env r r R T r Qhp, or 2 R SN r R T T, (12) r R hp where Q hp - heat flow used to heat one cylinder of the vessel, T hp - gas vessel wall temperature. 13

15 The first condition (12) corresponds to the situation where the power Q hp is known, and the second condition corresponds to the situation where the heating power is not limited and the temperature T hp is set. This temperature is maintained at the inner surface of the heat pipe due to the contact of its evaporation zone with a large heated body, such as an engine. To solve the set of equations (3, 5 8) with boundary conditions (1 12) the method of finite elements was chosen. The suggested simple model gives us a possibility to obtain the field of temperature and gas concentrations during charge-discharge (adsorption/desorption) procedures of the gas vessel. A set of calculations was performed for the high-surface area activated carbon fiber Busofit-M8 and wood-based carbon WAC 3- as promising gas sorption materials to design a hydrogen storage system. Figure 11 shows the plots of the volume storage density of hydrogen in the vessel (5 l) as a function of the temperature. It is evident that it is necessary to take into account the presence of the compressed gas in the adsorbed storage system at pressures MPa, the compressed fraction can be measured up to 3 5 %. The maximum volume storage density of hydrogen was 38 v/v at 77 K and 6 MPa for Busofit. It is important, that the whole amount of storage gas was about 18 v/v at 195 K. However, the WAC 3- sample ensured the value 12 v/v at 273 K.,v/v ρ, V/V WAC 3-1, P=6 MPa 2, P=3.5 MPa 1, P=6 MPa 2, P=3.5 MPa Busofit-M8 1, P=6 MPa 2, P=6 MPa 1, P=3.5 MPa 2, P=3.5 MPa T, K T, K T, K T, K Figure 11. Volume storage density of hydrogen (1 the adsorbed and compressed gases; 2 the adsorbed phase) vs. temperature for different pressure: P = 6 MPa and 3.5 MPa 6. Heat pipe based heat exchangers The efficient system to perform a sorbent bed thermal control during its charging/discharging is heat pipe based heat exchangers. Sorbents can be used as a compact sandwich with flat or cylindrical heat pipes being in good thermal contact with the surrounding or source of energy (for example, gas flame, electric heater). Various types of heat pipes: conventional heat pipes, heat pipe panels, loop heat pipes, vapourdynamic thermosyphons and other two phase evaporation/condensation heat transfer 14

16 technologies and equipment were developed and used for sorbent bed thermo control at Heat and Mass Transfer Institute. Choosing of appropriate type of heat pipes is complex problem, which depends on working conditions and limitations, temperature range, source of energy, necessity of recovery, cost etc. Heat pipes can easily be implemented inside sorption storage vessels. They are the most convenient thermal control devices for the solid and liquid sorption machines due to its flexibility, high thermal efficiency, cost-effectiveness, reliability, long operating life, simple manufacturing technology. Conventional heat pipes (Fig. 12) are convenient as heat transfer devices for sorption bed layer thermal control [15]. Q Figure 12. Conventional heat pipes schematic: 1 envelope, 2 porous wick, 3 vapour channel, 4 vapour, 5 liquid A very important feature of heat pipe (HP) is the ability to transport a large amounts of energy over the length of heat pipe with a small temperature drop by means of liquid evaporation at the heat pipe evaporator (heat source) and vapour condensation at the condenser (heat sink) and liquid movement in the opposite direction inside a wick by capillary force. Essential is a possibility to change the direction of a heat flow along the heat pipe in time and to use heat pipes for cooling and heating alternatively. Thermosyphons are a simple devise for thermal control of sorbent bed. Very suitable new design of thermosyphon, so called vapour-dynamic thermosyphon (Fig. 13), was especially developed for thermal regulating of continuous long horizontal objects. Q Figure 13. Water/SS "vapour-dynamic" thermosyphon: 1-electric heater; 2-boiler; 3-condenser; 4- feeding liquid tube; 5-vapour passage ; 6-trap for NCG (on the top of additional condenser); 7- water heat exchanger; a - water; b -vapour; c NCG 15

17 The advantages of this thermosyphon are [16]: 1) high heat transfer performance duty vapour and liquid flows separation, there are no interface friction losses; 2) low thermal resistance because of high velocity vapour flow in the gap between the condenser tubes make thin the liquid film on the condensing surface, enhancing heat transfer with condensation; 3) vapour flow in the gap evacuated the non-condensable gases to additional condenser (small volume at the end of vapour path from main condenser), that means the ability work with non-condensable gas presence; 4) the ability to transport of energy in horizontal direction, which is difficult to realise by conventional thermothyphons. Vapour-dynamic thermosyphons are capable to transport heat up to 1 kw and more for the distance some meters, which is difficult to realise by conventional thermosyphons, horizontally disposed Capillary pumped loops are attractive alternative for heat regulation [15]. The capillary pumped evaporator is used instead of the boiler. A capillary pumped evaporator is more flexible with the point of view of its orientation in space and is more compact. This principal difference of vapour generating gives the possibility to use the evaporator in higher level that condensers. For some applications flat heat pipe panels (HPP) have advantages over conventional cylindrical heat pipes, such as geometry adaptation, ability for localized heat dissipation and the maintenance of an entirely flat isothermal surface (Fig. 14). The liquid-vapour interface formed in capillary channels inside the heat pipe panel is capable to generate self-sustained thermally driven oscillations. Thin layer (several mm) of the sorbent between mini-fins on the outer side of the heat pipe panel ensures an advanced heat and mass transfer during the cycle adsorption/desorption. This device is filled partially with working fluid. The flow instabilities inside of this device are produced due to the heat input in one part of it and heat output from the another part by heating multi-channels (H = 2mm, L = 5 mm) at one end and simultaneously cooling the other end thus resulting in pulsating fluid. This heat input and output stimulates a heat transfer, as a combination of sensible and latent heat portions. The flow instabilities are a superposition of various underlying effects. Figure 14. Heat pipe panels (pulsating heat pipes) and cylindrical heat pipes for sorbent bed thermal control 16

18 In our experiments as an experimental set-up an aluminium multi-channel panel was chosen. The main parameters of flat aluminium heat pipe panel, developed in the Luikov Institute are: HPP width -7mm, HPP height - 7 mm, HPP length 7 mm, evaporator length - 98 mm, condenser length 5 mm, mass-, 43 kg. Propane was applied to fill the HPP and it is interesting as a working fluid with the point of view of its compatibility with all heat pipe envelopes and wick materials (aluminium, steel, stainless steel, copper, AL 2 O 3, etc). 7. Conclusions Original hydrogen sorption capacity and structural data for carbon materials were obtained. Empirical formulas for predicting of hydrogen sorption by carbons were offered. To improve the parameters of the gas storage systems the technology of carbon sorption capacity enhancement including additional activation was developed. The application of heat pipes in gas accumulator enables one to control the temperature of sorbent bed and provide optimum operational conditions. Different heat pipe devices for thermal control (cylindrical copper pipes with metal sintered powder as a wick, flat heat pipe panels, loop heat pipes with capillary pumped evaporators, vapour-dynamic thermosyphons and others) were developed and can be successfully integrated in sorption storage vessels. A simple theoretical model of cylindrical AG vessel with internal finned heater/cooler (HP based) was suggested. Application of sorption material based on activated carbon in solid sorption storage vessels has good perspective for developing a new type of gas storage systems. The designed AGS systems and modified carbon sorbents (BUSOFIT-M8, WAC 3-) are perspective for the hydrogen storage and for two-fuel automobile. 8. Acknowledgments Research supported by the Program of focused basic Researches Hydrogen Energy of National Academy of Sciences of Belarus, project No Hydrogen Nomenclature a, current, or nonequilibrium, adsorption, kg/kg; c, density of free gas, kg/m 3 ; C, specific mass heat of the sorbent skeleton, J/(kg-K); C g, specific mass heat of the free gas, J/(kg-K); C a, heat capacity of the adsorbed methane, J/(kg-K); E, activation energy, J/kg; G, mass flow rate of the gas flowing out of the cylinder, kg/sec, g/sec; g i, mass flow rate of the gas flowing out of the calculated cell of the cylinder, kg/sec; K s, preexponential factor in the approximate kinetic equation; m, dynamic coefficient of filling the cylinder; m e, unwithdrawn gas mass; M, mass of the gas in the cylinder, kg; M i, mass of the gas in the calculated cell, kg; N, number of calculated cells in the cylinder; P, pressure, Pa; q st, heat of phase transition, or isosteric sorption heat, J/kg; Q hp, power of heating of the entire cylinder, W; r and z cylindrical coordinates, m; R, outside radius of the cylinder shell, m; R, inside radius of the heating-element shell, m; r and r 1, inside and outside radii of the annular layer of the sorbent, m; R, gas constant, J/(kg-K); R p, 17

19 mean radius of the particles, mm; T hp, temperature at the inner surface of the heatingelement shell, K, C; 2S, finning step, m, mm; T, temperature, K, C; T, mean temperature of the sorbent layer, K; v, component of the velocity vector, m/sec; а specific volume of adsorbed media; V a, partial molar adsorption volume; V g, molar gasphase volume; z g, coefficient of gas compressibility;, coefficient of heat transfer, W/(m 2 -K);, porosity determined as a part of the volume occupied by the free gas (not bound by adsorption); 2, fin thickness, m, mm; eff, effective thermal conductivity of the sorbent layer, W/(m-K); s, density of the sorbent skeleton, kg/m 3 ;, total density of the free and adsorbed gases in the cylinder, kg/m 3 ; v, volume density of storage, nm 3 /m 3 ;, time, sec. Subscripts. eq, equilibrium conditions; a, adsorbate; cr, critical state; e, finite value; env, environment; hp, heat pipe;, initial value; s, sorbent; f, fin; t, transfer. 1. References 1. Hynek S, Fuller W, Bentley (1997) J. Hydrogen storage by carbon sorption. Int J Hydrogen Energy, 22, Carpetis C, Peschka W. A study on hydrogen storage by use of cryoadsorbents. Int J Hydrogen Energy, 5, Cheng H.M, Yang Q.H, Liu C. (22) Hydrogen storage in carbon nanotubes. Int J Hydrogen Energy, 27, Kuznetsov A.V, Vafai K. (1995) Analytical comparison and criteria for heat and mass transfer models in metal hydrides packed beds. Int J Hydrogen Energy, 38, Vasiliev L.L, Mishkinis D.A, Antukh A.A, Vasiliev L.L. Jr, (21) Solar Gas solid sorption refrigerator. Adsorption, 7, Vasiliev L.L, Kanonchik L.E, Mishkinis D.A, Rabetsky M.I. (2) Adsorbed natural gas storage and transportation vessels. Int J Therm. Sci., 39, Vasiliev L.L, Kulakov A.G, Mishkinis D.A, Safonova A.M, Luneva N.K. (24) Activated Carbon For Gas Adsorption. Proc. of III Int. Symposium Fullerene and Semifullerene Structures in the Condensed Media, Minsk, Belarus, Brunauer S, Emmertt P.H. Teller E. (1938) J Am. Chem. Soc., 6, Adsorbed Natural Gas Research Conducted by Atlanta Gas Light Adsorbent Group (AGLARG). Final report ( ) Gas Research Institute. Atlanta GRI- 95/68, Nijkamp M.G, Raaymakers J.E.M.J, van Dillen A.J, de Jong K.P. (21) Hydrogen storage using physisorption materials demands. Applied Physics A. Materials Science & Processing. A72, Vasiliev L.L, Mishkinis D.A, Kanonchik L.E, Khrolenok V.V, Zhuravlyov A.S. (1997) Activated carbon ammonia and natural gas adsorptive storage. Ext. Abs. 23 rd Biennial Conf. on Carbon. Carbon 97, Philadelphia,. Vol. 1, Vasiliev L.L, Kanonchik L.E, Mishkinis D.A, Rabetsky M.I. (2) A new method of Methane Storage and Transportation. Int J Enviromentally Conscious Design & Manufacting, 9,

20 13. Dubinin M.M. (196) The potential theory of adsorption of gases and vapors for sorbents with energetically nonuniform surfaces. Chem. Rev., 6, Vasiliev L.L, Kulakov A.G, Mishkinis D.A. (25) Activated carbon for gas adsorption. Proc of Int. Conference Solid State Hydrogen Storage Materials and Applications, Hyderabad, India Vasiliev L.L., Kulakov A.G. (23)Heat Pipe applications in sorption refrigerators, Low Temperature and Cryogenic Refrigeration. NATO Science Series II, Vol 99, Kluwer Academic Publishers, Vasiliev L.L. et al. (1985) Heat transfer Device. US Patent